Only one person was reported wounded and police evacuated three stations and cordoned off a bus with shattered windows, but the lunch-hour explosions caused major shock and disruption in the capital and were hauntingly similar to the July 7 bombings by four attackers.

The London police commissioner confirmed that four explosions took place in what he described as "a very serious incident."

"We've had four explosions -- four attempts at explosions," Metropolitan Police Commissioner Ian Blair said outside police headquarters at Scotland Yard.

"At the moment the casualty numbers appear to be very low ... the bombs appear to be smaller" than those detonated July 7.

He appealed to Londoners to stay where they were and said the transport system was effectively being shut down.

The minor explosions used detonators only, a BBC reporter said.

In addition, a Number 26 bus in Hackney Road in Bethnal Green had its windows blown out by a blast. There were no injuries.

Large areas around Warren Street, Oval and the Shepherd's Bush Hammersmith and City line Tube stations have been cordoned off. Four London Underground train lines were shut down.

Sky News TV reported that police were searching for a man with a blue shirt with wires protruding. In a memo to hospital staff, officers asked employees to look for a black or Asian male, 6 feet 2 inches tall, wearing a blue top with a hole in the back and wires protruding.

The explosions did not shut down the subway system, although three lines remained closed more than two hours later.

One witness told The Associated Press he smelled something similar to an electrical fire.

Police in chemical protection suits were seen preparing to enter the Warren Street Underground station. Sky News reported that police said no chemical agents were involved in the explosions.

British Prime Minister Tony Blair called off a press conference with visiting Australian leader John Howard and officials said he would be making a statement later after convening a crisis meeting of emergency officials and ministers.

"We can't minimises incidents such as this … we know why these things are done -- they're done to scare people, to frighten, to make them anxious and worried," Blair said.

"I think we've just got to react calmly and continue with our business as much as possible as normal," the prime minister added.

Back on the streets, train passengers who had fled reported panic and chaos below ground.

"I was in the carriage and we smelt smoke -- it was like something was burning," said Fosiane Mohellavi, 35, who was evacuated from a train at Warren Street.

"Everyone was panicked and people were screaming. We had to pull the alarm. I am still shaking," she told Britain's domestic Press Association.

Police were unable to confirm the cause of the alerts, which prompted the suspension of four subway lines.

Victoria Line passenger Ivan McCracken said he heard a traveller's rucksack had exploded on the Tube outside Warren Street station.

He told Sky News: "I was in a middle carriage and the train was not far short of Warren Street station when suddenly the door between my carriage and the next one burst open and dozens of people started rushing through. Some were falling, there was mass panic.

"It was difficult to get the story from any of them what had happened but when I got to ground level there was an Italian young man comforting an Italian girl who told me he had seen what had happened.

"He said that a man was carrying a rucksack and the rucksack suddenly exploded. It was a minor explosion but enough to blow open the rucksack.

"The man then made an exclamation as if something had gone wrong. At that point everyone rushed from the carriage."

Another witness on the same line, Abena Adofo, 23, said people were running into her carriage.

"I could just smell smoke and I saw lots of people panicking. I was just trying to be calm and get out. The smell of smoke was coming from the end of the carriage," she told Adofo. (AFP, AP, Reuters, BBC Online)